(1) Field of the Invention
This invention relates to an infrared absorbent. More specifically this invention relates to a novel infrared absorbent suitable for an optical fiber for absorbing near infrared rays or far infrared rays having a wavelength of 700 nm or more.
(2) Description of the Prior Art
Heretofore, various applications of an infrared absorbing material capable of selectively absorbing rays of a far infrared light of a near infrared light having a wavelength of 700 nm or more has been proposed. The following is five examples of conventional primary applications of the infrared absorbing material.
(1) Safelight filter for infrared-sensitive materials:
Recently, there have been developed many silver halide light sensitive materials (which will be hereinafter referred to as "light sensitive materials") which are sensitive to rays of a far infrared light or a near infrared light having a wavelength of 700 nm or more. That is, light sensitive materials are made to have an infrared sensitivity irrespective any distinction between black and white photographs or color photographs including a normal-type, instant-type and thermal developed-type photographs. These materials are useful for an artificial color photograph for resource search or to be exposable with a light emission diode capable of emitting a light in an infrared area.
Conventionally, a safelight filter for a panchromatic photosensitive material has been used as such infrared-sensitive materials.
(2) Control of growth of plants:
It has been long known that a so-called morphogenesis with regard to the growth and differentiation of plants such as the germination of a seed, the extension of a stem, the development of a leaf, the budding of a flower and the formation of tuber is influenced by light, and it has been studied as a photomorphogenesis.
If a plastic film capable of selectively absorbing rays having a wavelength of 700 nm or more is obtainable, it will be possible to control a spectral energy distribution of a light to adapt the above-mentioned principle to an actual productive cite, thereby providing a great progress and profit to an equipped agriculture site. For example, it is expected that earing time may be delayed or growth may be controlled by covering plants with a near-infrared absorbing film at a specific time to cut-off light having a wavelength of 700 nm or more. (See "Chemical Control of Plants", Katsumi Ineda, Vol. 6, No. 1 (1971))
(3) Cut-off of heat radiation:
Solar radient energy rays of a near infrared and an infrared area having a wavelength of 800 nm or more is absorbed by an object and converted to a thermal energy. In addition, a large part of its energy distribution is converged at a near infrared area having a wavelength of 800-2000 nm. Accordingly, a film capable of selectively absorbing rays of a near infrared light is remarkably effective for the cut-off of solar energy, and it is possible to suppress an increase in temperature in a room admitting visible light. Such a film may be adapted to a window of a house, office, store, automobile and airplane, etc. as well as a gardening green house. In particular, as to the green house, temperature control is very important, and if temperature is excessively elevated, plants will be greatly damaged which will finally result in withering. Accordingly, when the near infrared absorbing film is used, the temperature control may be rendered easy, and a new technique such as retarded cultivation in summer may be developed. A conventional heat radiation cutting-off material includes a thin metallic layer deposited on a surface of a plastic film or an inorganic compound, e.g., FeO dispersed in a glass.
(4) Cut filter of infrared rays noxious to tissues of human eyes:
Infrared rays contained in sun light or in light radiated by welding have a harmful influence to the tissues of human eyes. One of the primary applications of the infrared cut filter is an application to spectacles for protecting the human eyes from rays of light containing such harmful infrared rays, e.g., sunglasses and protecting glasses in welding.
(5) Infrared cut filter for a semiconductor light receiving element:
In another field where development of this kind of infrared absorbing plastics is most intensively desired, the infrared absorbing plastics are adapted to an infrared cut filter for a photosensor to make the spectral sensitivity of a semiconductor light receiving element such as silicon photo diode (which will be hereinafter referred to as SPD) approach to a relative spectral sensitivity curve.
Presently, SPD is mainly used as a light receiving element of a photosensor used in an automatic exposure meter for a camera or the like. FIG. 2 shows a graph of the relative spectral sensitivity curve and that of a relative value of an output of SPD to each wavelength.
In order to use SPD for an exposure meter, it is required to cut-off light in an infrared area which is not sensitive to human eyes and to make the spectral sensitivity curve of SPD shown in FIG. 2 approach the relative spectral sensitivity curve. Particularly, as an output of SPD is large to the light having a wavelength of 700-1100 nm, and the eyes are insensitive to such a light, this is one of factors of malfunction of the exposure meter. Therefore, if it is possible to use an infrared absorbing plastic film suppressing an absorption of a visible light area, while permitting an absorption of an infrared light area in the entire range of 700-1100 nm, light transmittance in a visible area may be increased and an output of SPD may also be increased. Thus, it will be possible to apparently remarkably improve the performance of the exposure meter.
Conventionally, this kind of photosensor has been practically used by mounting an infrared cut filter made of glass containing an inorganic infrared absorbent to a front surface of SPD.
Moreover, in general the organic dyestuff infrared absorbents in the prior art are almost always unsatisfactory in practical use because of their low light fastness and heat fastness.
Further, regarding the above-mentioned applications, filter materials as used have the following shortcomings.
The safelight of filter for the panchromic photosensitive material in the afore-mentioned applications (1) permits a green light having a high luminosity factor to be partially transmitted, and also permits a large quantity of infrared light to be transmitted in cause fogging. For this reason, such a safelight filter has not been able to achieve its object for infrared sensitive materials.
In the applications (3), the metallic layer deposited plastic film or the FeO dispersed glass functions to intensively absorb not only infrared light but also visible light to cause reduction in inside luminance. For this reason, such a plastic film or glass is not suitable for agricultural uses because of the lack of an absolute quantity of sunshine. Especially, the filter material for growth control of plants in the applications (2) is required to selectively absorb a light having a wavelength of 700-750 nm, and therefore the metallic layer deposited film is quite unsuitable for such an object.
Furthermore, in the applications (5), the infrared cut filter using the infrared absorbent containing an inorganic substance is relatively fast to heat and light, but light transmittance in a visible area is low. To cope with this, the sensitivity of SPD was intended to be increased. However, an increase in the sensitivity of SPD results in an increase in leak current to cause a malfunction of the photosensor, resulting in a big problem in reliability. Additionally, since the infrared cut filter contains an inorganic substance, there is a lack in the flexibility in the production of a photosensor and a difficulty in improving the production process. Further, the infrared cut filter containing an inorganic substance causes a high production cost which results in a great increase in the cost of the photosensor.
In this manner, although the photosensor using the conventional cut filter containing an inorganic substance has a spectral sensitivity similar to the spectral luminous efficiency curve, it has a remarkable defect in such a viewpoint as a reduction in the operational performance, an increase in the production cost and a difficulty in improving the production process.
Moreover, the conventional near-infrared absorbing plastic film containing the infrared absorbent of a complex containing quaternary ammonium group does not have a sufficient solubility of the infrared absorbent to an organic solvent, which was a restriction in preparing a thin plastic film layer.
In other words, the SPD filter as mentioned above is desired to have a very small thickness and a good absorption efficiency of infrared rays. To this end, it is necessary to disperse a large quantity of infrared absorbent in resin. Therefore, the infrared absorbent having a small solubility to an organic solvent has not met the above requirements.
Furthermore, a conventional near-infrared absorbing plastic film containing a metal complex as an infrared absorbent has a soft wavelength of absorption maximum, and therefore it was unsuitable for application to a light receiving element of a semiconductor laser which is increasing its uses.